Conventional ultrasonic transducers for medical applications are constructed from one or more piezoelectric elements sandwiched between a pair of backing/matching layers. Such piezoelectric elements are constructed in the shape of plates or rectangular beams bonded to the backing and matching layers. The piezoelectric material is typically lead zirconate titanate (PZT), polyvinylidene difluoride (PVDF), or PZT ceramic/polymer composite.
Piezoelectric ceramics are formed by conventional processes, such as dry pressing, casting or extrusion, and then sintered. After sintering, the ceramic elements are machined to the desired dimensions and then plated to form electrodes. The final step is to subject the ceramic elements to a process known as "poling". In this process, the constituents of the ceramic material are physically reorganized by heating the material to a temperature in excess of the Curie temperature while maintaining an electric field across the material. The electric field organizes some atoms into electric domains that produce the piezoelectric effect. This reorganization is retained when the material is quenched.
One type of ultrasonic transducer element is made of composite material comprising piezoelectric ceramic material surrounded by a piezoelectrically passive polymer matrix. The combination of a piezoelectric ceramic with a polymer provides a flexible or shapable material with good piezoelectric properties.
When combining materials, in addition to selecting component phases which have the desired properties, one must couple the component phases to each other in an optimal manner. Connectivity of the individual phases is a critical parameter in composites designed for use as piezoelectric transducers because connectivity controls the electric flux pattern as well as the mechanical properties. Each phase in a composite may be self-connected in zero, one, two or three dimensions. For diphasic composites, there are ten connectivities: 0-0, 0-1, 0-2, 0-3, 1-1, 1-2, 1-3, 2-2, 2-3 and 3-3, where the first number in each pair indicates the number of dimensions of connectivity for the first component phase of a diphasic composite and the second number in each pair indicates the number of dimensions of connectivity for the second component phase of that diphasic composite.
The piezocomposite structures most commonly used in ultrasonic transducers are the 1-3 and 2-2 structures. For example, an exemplary 1-3 structure comprises a multiplicity of mutually parallel, spaced PZT rods embedded in a matrix of conformal polymer filler material. In contrast, the 2-2 structure comprises alternating layers of piezoelectric ceramic and polymer. The conformal polymer filler materials used in the design of piezocomposite resonators have historically been piezoelectrically passive.
There is a need for a piezocomposite material having improved piezoelectricity constant, dielectric constant and electromechanical coupling factor as compared to conventional piezocomposite materials used in ultrasonic transducers.